The present invention relates to apparatus and a method for determining whether line voltage is present in an alternating current power cord, and in particular to such a sensor that can be mounted on a power cord at any point along its length and which provides an easily viewed indication of whether alternating voltage is present in the power cord.
Consumers of electric power have long been inconvenienced with the problem of determining whether there is electric power going to an appliance or device that they are about to use. Traditionally they just switch on the appliance or device to see if it works. However, not all appliances can be determined to have a supply of electrical power at the moment the power switch is turned on.
Examples of this are coffee makers, soldering irons, and clothes irons that don't have visual power indicators. The voltage indicator described in the present disclosure provides a clear and almost instantaneous indication of whether power is available.
Various devices have been available previously for determining and indicating whether voltage is available in power cables and extension cords, but most have not been particularly satisfactory.
For example, U.S. Pat. No. 5,424,630 to Vazquez (1995) discloses a voltage indicator that has a knife blade as one electrical contact and an alligator clip as the other electrical contact of the voltage indicator. Two disadvantages of this indicator are readily apparent. The knife blade contact is used to cut through the insulation on the wire, thus permanently damaging it, and the alligator clip needs a grounded electrical terminal to be clipped on to complete the circuit.
U.S. Pat. No. 5,428,288 to Foreman et al. (1995) and U.S. Pat. No. 2,449,150 to Schnoll (1948) are for related indicators. In both of these patents a thin electrical connecting wafer is slid over the end of a standard male electric power plug. A common disadvantage of both of these inventions is that the thin wafer decreases the quality of the electrical connection of the male plug to the power outlet. The Foreman et al. patent also requires the use of a computer to sense a power failure while in the Schnoll patent the indicator can only be read at the male plug and not at anywhere along the length of the power cord as might be more convenient.
U.S. Pat. No. 4,829,289 to Kallman et al. (1989) and Canadian Patent No. 728,300 to Windsor (1966) are for related voltage indicators. The patents disclose an illuminated plug that can be installed between a normal power outlet and the male plug at the end of the power cord. Both of these devices have the disadvantage of being impractical for use at a power receptacle behind a piece of furniture. The device described in the Windsor patent has the additional disadvantage that an appliance or device connected through the device needs to be turned on to complete a path through the illuminated indicator in order for the device to provide a useful indication.
Swiss Patent No. 221,666 to Gerber (1942) and Canadian Patent No. 474,305 to Simmons (1951) disclose similar voltage indicators. Both patents disclose screwdriver or pen type voltage indicators that use a return path to ground through a human being. Both devices have the disadvantage of needing to make physical contact between a probe tip and a conductor of a power cord being considered.
Atherton U.S. Pat. No. 7,893,686 (2011) discloses a voltage indicator for use with an ordinary single-phase AC power cord of either the grounded or the ungrounded type, in which capacitive coupling takes place between an energized conductor of the power cord and a first terminal of an electronic display. A circuit for the electronic display is completed by a human touching a conductive element of the device, in response to which the display indicates whether voltage is present in the power cord.
Canadian Patent No. 993,952 to Friesen (1976) discloses a power cord voltage indicator in which metallic contacts pierce the conductors of the power cord under test. The piercing of the power cord conductors has the disadvantage of doing permanent damage to the power cord insulation as well as the disadvantage of being unusable on power cords that are completely enclosed in a protective outer sheath.
U.S. Pat. No. 5,095,265 to Schweitzer Jr. (1992) is for a coaxial power cord voltage indicator in which capacitive sensor must pass through the outer conductive jacket of the coaxial cable in order to be operative. The patent is limited to use only in coaxial cables in which an outer sheath is electrically conductive and can act to provide a ground return path.
Other patents to Schweitzer Jr. for related devices are U.S. Pat. No. 5,070,301 (1991), U.S. Pat. No. 5,077,520 (1991), and U.S. Pat. No. 5,363,088 (1994). These voltage indicators, however, are all intended for sensing voltage in coaxial cables with a ground return on an outer coaxial conductor.
U.S. Pat. No. 6,157,184 (2000) to Atherton discloses a voltage indicator for similar application.
What is desired, then, is a device that can be applied simply to a conventional alternating current power cord, at any chosen position along the power cord, and a method for sensing and providing a visible indication of whether ordinary alternating current voltage is present in the power cord. Such a device should not physically damage the power cord on which it is mounted, should not require any power source, such as a battery, of its own, and should require only a minimal amount of power, obtained from the power cord being observed.